1. Field of the Invention
This invention relates to medical devices and methods. More specifically, the invention relates to intervertebral disc prostheses.
Back pain takes an enormous toll on the health and productivity of people around the world. According to the American Academy of Orthopedic Surgeons, approximately 80 percent of Americans will experience back pain at some time in their life. In just the year 2000, approximately 26 million visits were made to physicians' offices due to back problems in the United States. On any one day, it is estimated that 5% of the working population in America is disabled by back pain.
One common cause of back pain is injury, degeneration and/or dysfunction of one or more intervertebral discs. Intervertebral discs are the soft tissue structures located between each of the thirty-three vertebral bones that make up the vertebral (spinal) column. Essentially, the discs allow the vertebrae to move relative to one another. The vertebral column and discs are vital anatomical structures, in that they form a central axis that supports the head and torso, allow for movement of the back, and protect the spinal cord, which passes through the vertebrae in proximity to the discs.
Discs often become damaged due to wear and tear or acute injury. For example, discs may bulge (herniate), tear, rupture, degenerate or the like. A bulging disc may press against the spinal cord or a nerve exiting the spinal cord, causing “radicular” pain (pain in one or more extremities caused by impingement of a nerve root). Degeneration or other damage to a disc may cause a loss of “disc height,” meaning that the natural space between two vertebrae decreases. Decreased disc height may cause a disc to bulge, facet loads to increase, two vertebrae to rub together in an unnatural way and/or increased pressure on certain parts of the vertebrae and/or nerve roots, thus causing pain. In general, chronic and acute damage to intervertebral discs is a common source of back related pain and loss of mobility.
When one or more damaged intervertebral discs cause a patient pain and discomfort, surgery is often required. Traditionally, surgical procedures for treating intervertebral discs have involved discectomy (partial or total removal of a disc), with or without fusion of the two vertebrae adjacent to the disc. Fusion of the two vertebrae is achieved by inserting bone graft material between the two vertebrae such that the two vertebrae and the graft material grow together. Oftentimes, pins, rods, screws, cages and/or the like are inserted between the vertebrae to act as support structures to hold the vertebrae and graft material in place while they permanently fuse together. Although fusion often treats the back pain, it reduces the patient's ability to move, because the back cannot bend or twist at the fused area. In addition, fusion increases stresses at adjacent levels of the spine, potentially accelerating degeneration of these discs.
In an attempt to treat disc related pain without fusion, an alternative approach has been developed, in which a movable, implantable, artificial intervertebral disc (or “disc prosthesis”) is inserted between two vertebrae. A number of different intervertebral disc prostheses are currently being developed. For example, the inventors of the present invention have developed disc prostheses described in U.S. patent application Ser. Nos. 10/855,817 and 10/855,253, previously incorporated by reference. Other examples of intervertebral disc prostheses are the LINK® SB CHARITTÉ™ disc prosthesis (provided by DePuy Spine, Inc.) MOBIDISK™ disc prosthesis (provided by LDR Medical), the BRYAN™ cervical disc prosthesis (provided by Medtronic Sofamor Danek, Inc.), the PRODISC™ disc prosthesis or PRODISC-C™ disc prosthesis (from Synthes Stratec, Inc.), and the PCM™ disc prosthesis (provided by Cervitech, Inc.).
To insert an artificial intervertebral disc prosthesis, and indeed for performing most disc-related surgeries, it is typically necessary to gain access to the disc and the intervertebral space from an anterior to posterior direction (i.e., through the front of the patient), to avoid coming in contact with the spinal cord. Thus, surgical procedures on a disc are typically approached anteriorly through the neck or abdomen, depending on which disc (or discs) is being repaired. Methods for inserting a disc prosthesis generally involve removing the damaged disc, preparing the surfaces of the two vertebral bones to receive the prosthesis, spreading the two vertebrae apart using one or more spreading devices, and inserting the prosthesis into the space between the two vertebrae. Examples of such methods are described in U.S. Pat. Nos. 6,478,800, 6,235,030, 6,652,533, 6,689,132, 6,261,296 and 6,666,866, and in U.S. Patent Application Nos. 2001/0031969, 2001/0029377, 2003/0153916, 2002/0198532, 2004/0024407, 2003/0216737, 2003/0204261, 2003/0135220 and 2003/0014114. Due to the invasive nature of such procedures, one important goal is to reduce invasiveness, thus causing as little trauma to tissues surrounding the surgical site as possible.
The main drawback of currently available methods for inserting intervertebral disc prostheses is that they require a larger amount of spreading apart (or “distraction”) of the two vertebrae than is optimal. Over-distraction is necessary when using such methods because it is important to insert the disc prosthesis all the way into the disc space, to position the center of rotation of the prosthesis closer to the posterior portion of the vertebrae than to the anterior portion. This allows the vertebrae to move as they were intended and avoids placing undue strain on the facet joints of the vertebrae or on other structures. To push a prosthesis toward the back of a disc space, however, it is typically necessary to spread the two vertebrae apart widely, since the anterior portion of the disc space is usually wider (or higher) than the posterior portion. In some methods, an artificial disc is placed by placing a first endplate into the space, placing a second endplate into the space, and then spreading the vertebrae wide enough to wedge a core in between the two endplates. In other methods, the vertebrae are spread apart as far as practicable, the whole prosthesis is inserted while one or more spreading devices are in place, and not until the prosthesis is completely inserted is the spreading device removed. In either case, as well as in other currently available methods, the two vertebrae adjacent the prosthesis are typically spread farther apart than would be desirable for a longer amount of time than would be desirable. In some cases, the posterior longitudinal ligament (PLL) is released, or “cut,” to enable sufficient distraction for disc placement.
Distracting vertebrae can damage muscles, ligaments, nerves and/or other tissues in and around the vertebral column. Such damage may actually cause the patient to experience as much, or even more, pain after surgery than was caused by the original disc problem.
Therefore, as the use of intervertebral disc prostheses increases, an increasing need exists for improved methods and apparatus for inserting such prostheses. Ideally, such intervertebral prosthesis insertion methods and devices would provide for insertion of a prosthesis a desired distance into an intervertebral space while reducing the need for intervertebral distraction, thus preventing or at least reducing trauma to surrounding tissues. Also ideally, such insertion methods and devices would be relatively simple and easy to use, thereby reducing the overall invasiveness of the procedure. At least some of these objectives will be met by the present invention.
2. Description of the Background Art
A number of exemplary intervertebral disc prostheses are listed above. Published US patent applications 2002/0035400A1 and 2002/0128715A1 describe disc implants which comprise opposing plates with a core between them over which the plates can slide. The core receives one or more central posts, which are carried by the plates and which locate in opposite ends of a central opening in the core. Such arrangements limit the load bearing area available between the plates and core.
Other patents related to intervertebral disc prostheses include U.S. Pat. Nos. 4,759,766; 4,863,477; 4,997,432; 5,035,716; 5,071,437; 5,370,697; 5,401,269; 5,507,816; 5,534,030; 5,556,431; 5,674,296; 5,676,702; 5,702,450; 5,824,094; 5,865,846; 5,989,291; 6,001,130; 6,022,376; 6,039,763; 6,139,579; 6,156,067; 6,162,252; 6,315,797; 6,348,071; 6,368,350; 6,416,551; 6,592,624; 6,607,558 and 6,706,068. Other patent applications related to intervertebral disc prostheses include U.S. Patent Application Publication Nos. 2003/0009224; 2003/0074076; 2003/0191536; 2003/0208271; 2003/0135277; 2003/0199982; 2001/0016773 and 2003/0100951. Other related patents include WO 01/01893A1, EP 1344507, EP 1344506, EP 1250898, EP 1306064, EP 1344508, EP 1344493, EP 1417940, EP 1142544, and EP 0333990.